1. Field of the Invention
The field of the invention is posterior chamber intraocular lenses. More particularly, the field of the invention is such lenses which are implanted either when or after an intracapsular procedure has been performed and where, therefore, the posterior capsule is no longer present to serve as a fixation platform.
2. Description of the Prior Art
There are generally two types of intraocular lens implants in use today--those implants which are positioned in front of the iris (anterior chamber lens implants) and those implants which are positioned behind the iris (posterior chamber lens implants). Anterior chambers lenses are simpler to implant than posterior chamber lenses, however, posterior chamber lenses present several advantages over anterior chamber lenses. One of those advantages concerns the preservation of the endothelial cells of the cornea.
The endothelium of the cornea is a one cell layer thick membrane which acts to pump fluids out of the cornea, thereby keeping the cornea clear. Endothelial cells of the cornea do not regenerate when they are destroyed, rather, adjacent endothelial cells expand to fill the area vacated by the destroyed cells. When the number of cells remaining reaches a critically low level, the patient may suffer from complications resulting from a lack of sufficient endothelial cells. Thus an operating surgeon ordinarily takes great care to avoid contact with endothelial cells during an intraocular implantation procedure, since if any cells are touched by a surgical instrument or suture material, they are destroyed and will not be replaced.
Because of its position in front of the iris, an anterior chamber lens implant is more likely to come in contact with endothelial cells of the cornea, in which case those cells will be destroyed. This is true not only during the implantation procedure, but while the implant is functionally in place as well. Furthermore, an hydrostatic charge on the surface of a plastic anterior chamber lens tends to pull endothelial cells away from the cornea.
Anterior chamber lenses possess another disadvantage verses posterior chamber lenses in that anterior chamber lenses can potentially block the filtration of fluids through the angle of the eye. This blocking of the natural filtration process of the eye creates an increased likelihood of glaucoma.
There are basically two general types of cataract removal techniques employed today--extracapsular and intracapsular procedures. In an extracapsular procedure, the capsule behind the lens (posterior capsule) is left intact, whereas in an intracapsular procedure, the posterior capsule is extracted in the course of the procedure.
After an extracapsular procedure, the posterior capsule can readily serve as a stable site for the lodging of a posterior chamber intraocular lens implant within the eye. Posterior chamber lenses generally take advantage of this fact by including haptics which extend from the lens portion of the implant into the ciliary sulcus of the eye where they maintain the lens in position and where there is no risk of the lens floating back into the eyeball. With an intracapsular procedure, however, the capsule is no longer in place to retain a posterior chamber lens within the sulcus. The implant is therefore normally sutured into place by the surgeon. After an intracapsular procedure, lenses have been sutured into place by either tying the ciliary sulcus haptics to the iris or by using the positioning holes of the lens as suture sites. Also, various types of lenses have been specifically designed which have provided for some type of attachment to the iris. Examples of such lenses can be found in U.S. Pat. Nos. 3,866,249; 3,991,426; 4,110,848; 4,198,714; 4,304,012; 4,404,694; 4,316,291; and 4,336,582. Where posterior chamber lens implants have been so attached, however, they have tended to cause damage to the iris by rubbing against the iris tissue as the iris expands and contracts during focusing.
Thus prior to the present invention, where there has been no posterior capsule present to serve as a fixation platform for a posterior lens implant, the operating surgeon has either (1) implanted an anterior chamber lens, which tends to destroy irreplaceable endothelial cells of the cornea and increases the risk of glaucoma, or (2) attached a posterior chamber lens to the iris in one of the above ways which has tended to cause damage to the iris tissue.